Concrete Calculator

1. Pick what you are pouring: a slab or footing, a round column or tube, or a set of post holes. Choose imperial (feet and inches) or metric (meters and centimeters).
2. Enter the dimensions for that shape, then set a waste allowance for spillage and over-dig.
3. Read the volume in cubic yards, cubic feet and cubic meters, then pick a bag brand and size to see how many bags you would need. Everything updates as you type.

Here is what each figure is for:

Concrete is sold by volume, and in the United States that volume is the cubic yard: a cube one yard on every side, equal to 27 cubic feet or about 0.765 cubic meters. A single cubic yard covers roughly 81 square feet at four inches thick, which is why slabs are quoted in yards rather than bags once they pass a certain size.

Everywhere else, concrete is ordered in cubic meters. The two units describe the same material, so this calculator shows cubic feet, cubic yards and cubic meters together, letting you talk to a US ready-mix plant and an international supplier without redoing the math.

The tool finds the volume of the shape you chose, converts every dimension to a common unit first, then adds your waste allowance and divides by the bag yield for a bag count.

1. Put every dimension into feet. In imperial, lengths are feet and thickness, diameters and hole sizes are inches; in metric, lengths are meters and the rest are centimeters.
2. Apply the shape formula above for the raw volume in cubic feet.
3. Add the waste percentage, then convert to cubic yards (divide by 27) and cubic meters.
4. Divide the volume by the per-bag yield and round up for the number of bags.

Bag counts use the verified yields below, for example 0.60 cubic feet for an 80-pound bag and 0.45 for a 60-pound bag, so the estimate matches the bag you actually buy.

Take a 10 by 10 foot slab, 4 inches thick. The thickness is 4 divided by 12, or 0.333 feet, so the volume is 10 times 10 times 0.333, which is about 33.33 cubic feet.

Dividing 33.33 by 27 gives roughly 1.235 cubic yards, or about 0.94 cubic meters. At 0.60 cubic feet per 80-pound bag that is round up (33.33 / 0.60) = 56 bags of Quikrete Concrete Mix, which is exactly the point where a ready-mix delivery starts to look easier and cheaper than hauling bags.

Adding a 10 percent waste allowance raises the order to about 36.67 cubic feet, so plan on a few extra bags. Drop the thickness to a 3-inch path and the same footprint needs only about 0.92 cubic yards, showing how strongly thickness drives the total.

Beyond flat slabs, this calculator also sizes two round pours that trip people up: columns cast in a tube form, and the concrete that sets fence or deck posts.

Round Columns and Form Tubes

A column or cardboard form tube is a cylinder, so the volume is pi times the radius squared times the height. A 12-inch diameter tube 10 feet tall holds pi times 0.5 squared times 10, about 7.85 cubic feet, or roughly 0.29 cubic yards. Enter the diameter in inches (or centimeters) and the height in feet (or meters).

Post Holes and Fence Posts

For a post, the concrete fills the hole around the post, so the tool subtracts the post from the hole. Take 4 posts, each in a 9-inch diameter hole 30 inches deep, with a 3-inch square post:

• Hole volume: pi times 0.375 squared times 2.5 = 1.10 cu ft (using 1.1045).
• Post volume: 0.25 squared times 2.5 = 0.15625 cu ft.
• Per post: about 0.948 cu ft; across 4 posts, 3.793 cu ft.
• Bags: at 0.375 cu ft per Quikrete 50-pound bag, round up (3.793 / 0.375) = 11 bags.

Setting several posts? You can also square up rectangular layouts with the aspect ratio calculator and size the plot with the land area calculator.

Bagged concrete comes in a few standard weights, and the published yield decides how many you carry. These figures are read directly from the Quikrete and Sakrete data sheets.

Because a cubic yard is 27 cubic feet, dividing 27 by the bag yield gives the bags per yard. Sakrete publishes a cubic-foot yield only for its 40, 60 and 80-pound High-Strength bags, so for a 50 or 90-pound order the tool points you to Quikrete or to a published Sakrete size rather than guessing. Once you are near a full yard, the bag count and the effort both climb quickly.

The footprint is only part of the story; a few factors move the real quantity up or down.

Thickness and Depth

Volume scales directly with depth, so going from 4 to 6 inches adds fifty percent, often the single biggest swing in the order.

Waste and Spillage

Some concrete sticks to the mixer, spills, or fills low spots, so a 5 to 10 percent allowance keeps you from coming up short. Set that figure in the waste field; the percentage calculator helps if you want to check it against a quote.

Subgrade Evenness

An uneven or over-excavated base quietly swallows extra concrete beyond the nominal thickness you planned.

Shape and Edges

Thickened edges, footings and curves add volume that a plain length-by-width figure misses. For separate finishing products, see the self-leveling calculator and the joint filler calculator.

Small Repairs and Posts

For a few cubic feet, bagged concrete you mix on site is cheaper and avoids a delivery minimum. Post holes and small footings almost always suit bags.

Slabs Near or Above a Cubic Yard

Once the volume reaches roughly one cubic yard, ready-mix is usually cheaper per unit and far less labor than dozens of bags.

Budgeting the Pour

Knowing the cubic yards up front lets you price both options and compare them against the delivery cost before you commit. If you are financing a larger project, the loan calculator and mortgage calculator help you work the payments into the budget, and the drywall estimate guide covers the interior finishing that often follows a pour.

1. Mixing Units

Keep every dimension in the right unit for the mode. This tool converts thickness and hole sizes for you, but only if you pick the right imperial or metric mode.

2. Forgetting a Waste Allowance

Order the calculated volume plus about 10 percent so a small overrun does not stop the pour.

3. Ignoring Thickened Edges

Footings and edge beams hold more concrete than the flat slab figure suggests.

4. Forgetting to Subtract the Post

In post-hole mode the concrete fills the hole around the post, so a fat post noticeably cuts the concrete you need. Enter the post size.

5. Rounding Bags Down

Always round the bag count up; running one bag short mid-pour ruins the finish.

6. Treating Bag Yield as Exact

Yields vary a little by product, so read the bag and adjust if it differs from the data-sheet figures.

The volume math is exact for the shapes offered, and the conversions use standard factors, but real pours carry variables the geometry cannot see.

Post-hole depth in particular is a code and frost-line question, not a formula; treat the 3-times-width and one-third-height guides as starting points and follow your local requirements.

Multiply the dimensions for your shape, keeping them in the same unit. For a slab it is length times width times thickness; for a round column it is pi times the radius squared times the height. This calculator does that and converts the result into cubic yards, cubic meters and an estimated bag count.

There are 27 cubic feet in a cubic yard, because a cubic yard is 3 by 3 by 3 feet. Divide cubic feet by 27 to get cubic yards.

About 45 of the 80-pound bags, 60 of the 60-pound bags, or 90 of the 40-pound bags, based on the Quikrete and Sakrete data-sheet yields.

An 80-pound bag of Quikrete or Sakrete concrete yields 0.6 cubic feet of mixed concrete, enough to cover about 1.8 square feet at four inches thick.

Enter the number of posts, the hole diameter and depth, and the post shape and size. The tool finds each hole volume, subtracts the post, and multiplies by the number of posts. Four 9-inch holes 30 inches deep with 3-inch square posts need about 3.79 cubic feet, or 11 Quikrete 50-pound bags.

A round column or form tube is a cylinder: pi times the radius squared times the height. A 12-inch tube 10 feet tall holds about 7.85 cubic feet, roughly 0.29 cubic yards. Switch the mode to Round column and enter the diameter and height.

For small jobs under about a cubic yard, such as posts and small footings, bags are cheaper and simpler. At or above a cubic yard, ready-mix is usually cheaper per unit and much less labor.

Add about 5 to 10 percent for spillage, an uneven subgrade and thickened edges, so a small overrun never halts the pour. Set this in the waste allowance field.

Yes. Switch to metric to enter lengths in meters and thickness, diameters and hole sizes in centimeters, and read the result in cubic meters.

No. The calculation runs in your browser, and nothing you type is sent anywhere unless you Save a result, which stays on this device only.

• Quikrete Concrete Mix No. 1101 data sheet (per-bag yields: 40=0.30, 50=0.375, 60=0.45, 80=0.60, 90=0.675 cu ft) (Quikrete, read 2026-10-05).
• Sakrete High-Strength Concrete Mix product page (yields: 40=0.30, 60=0.45, 80=0.60 cu ft) (Sakrete, read 2026-10-05).
• Quikrete Fast-Setting Concrete Mix No. 1004-50 data sheet (post-setting guidance: hole about 3x the post diameter, depth about one third of the post height) (Quikrete, read 2026-10-05).
• Approximate conversions from US customary to metric measures (cubic foot, cubic yard, cubic meter) (National Institute of Standards and Technology).